This invention relates to rubber compositions comprised of novel blends of styrene-rich segmented styrene/isoprene/butadiene terpolymer elastomer and at least one additional elastomer and to tires with treads comprised of such rubber composition. Such tires may exhibit increased traction and reduced rolling resistance.
Pneumatic rubber tires conventionally have treads of various elastomer based compositions which are subject, under operating conditions, to considerable dynamic distortion and flexing as well as abrasion due to, for example, skidding and scuffing as is well known to those having skill in such art.
Historically, a tire tread may be composed of, for example, one or more elastomers such as, for example, cis 1,4-polyisoprene, cis 1,4-polybutadiene and styrene/butadiene (SBR) copolymers. Where enhanced traction of the tread is desired, elastomers such as, for example, 3,4-polyisoprene or SBR polymers might be utilized in the tread rubber blend, depending somewhat upon the overall choice of elastomers for the tire tread.
Historically, where enhanced abrasion resistance is desired, cis 1,4-polybutadiene rubber may be a component of the tire tread rubber composition. It is believed that such utilization of such elastomers is well known to those having skill in such art.
For this invention, a novel elastomer blend is desired to enhance, or promote, a tire tread""s traction.
In the description of this invention, the terms xe2x80x9ccompoundedxe2x80x9d rubber compositions and xe2x80x9ccompoundsxe2x80x9d, where used refer to the respective rubber compositions which have been compounded with appropriate compounding ingredients such as, for example, carbon black, oil, stearic acid, zinc oxide, silica, wax, antidegradants, resin(s), sulfur and accelerator(s) where appropriate. The terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d may be used interchangeably. The amounts of materials, or ingredients, for a rubber composition are usually expressed in parts of material per 100 parts of elastomer by weight (phr).
In accordance with this invention, a rubber composition is provided which is comprised of, based upon 100 parts by weight rubber (phr), (A) about 25 to about 75 phr of styrene-rich segmented styrene/isoprene/butadiene terpolymer elastomer comprised of styrene/butadiene segments and polyisoprene segments and, correspondingly, about 25 to about 75 phr of at least one additional elastomer selected from at least one of cis 1,4-polyisoprene, styrene/butadiene copolymer, styrene/isoprene copolymer and 3,4-polyisoprene.
It is preferred that the SBR segments are styrene-rich segments which contain about 20 to about 50, alternately about 25 to about 45, weight percent bound styrene. Accordingly, the terpolymer preferably contains from about 15 to about 35, alternately about 20 to about 30, weight percent bound styrene.
In practice, it is preferred that the content of the styrene/butadiene (SBR) copolymer segments is in a range of about 15% to about 50% of the terpolymer and, correspondingly, the content of the polyisoprene segments is in a range of about 85% to about 50% of the terpolymer.
In practice, it is preferred that said segmented styrene/isoprene/butadiene terpolymer is characterized by having two Tg""s, namely a first Tg in a range of about +10 to about xe2x88x9230xc2x0 C. and a second Tg in a range of about xe2x88x9250 to about xe2x88x9270xc2x0 C. Accordingly, the first and second Tg""s are spaced apart by at least 20xc2x0 C. Because of the significant spatial difference between the two Tg""s, it is considered herein that the SBR and the polyisoprene segments of the terpolymer are substantially incompatible with each other.
It is considered herein that the relative incompatibility of segments within the terpolymer is significant because it allows each polymer segment to provide its inherent individual property to the segmented polymer. Accordingly, the styrene rich segment of the terpolymer is, thereby, able to more individually contribute to the traction aspect of the rubber composition.
In further accordance with this invention, such a rubber composition is provided which contains about 20 to about 120 phr of reinforcing filler selected from (1) rubber reinforcing carbon black or (2) rubber reinforcing carbon black and amorphous silica where said carbon black is present in an amount of about 5 to about 80 phr, preferably about 30 to about 60 phr and the amorphous silica is present in an amount of about 5 to about 80, alternatively about 15 to about 55, phr so long as the total of said carbon black and silica fillers is within the aforesaid range of about 20 to about 120, preferably about 35 to about 85 phr.
A significant aspect of this invention is the use of a segmented styrene/isoprene/butadiene terpolymer elastomer in a tire tread rubber composition, particularly together with one or more of cis 1,4-polyisoprene, styrene/butadiene copolymer, styrene/isoprene copolymer and 3,4-polyisoprene elastomers.
Preferably the cis 1,4-polyisoprene rubber is natural rubber, although synthetic cis 1,4-polyisoprene rubber may usually be acceptable.
It is considered herein that use of a segmented styrene/isoprene/butadiene elastomer is particularly beneficial for promoting traction, or skid resistance, for a tire tread rubber composition because this polymer contains styrene-rich SBR segments that are chemically bound with high cis 1,4-polyisoprene segments (e.g. polyisoprene segments of about 80 percent cis 1,4-configuration).
In particular, for the relatively styrene-rich segmented styrene/isoprene/butadiene terpolymer elastomer, it is preferred that it is composed of about 20 to about 50 weight percent units derived from styrene, about 10 to about 50 weight percent units derived from isoprene and about 30 to about 70 weight percent units derived from 1,3-butadiene.
This aspect of the structure is considered herein to be important because it promotes the incompatibility of the SBR and polyisoprene segments to provide the desired cured rubber properties.
Silica reinforcement, where used, is conventionally used in conjunction with a coupling agent to couple the silica to the elastomer(s), thus, enhancing its effect as reinforcement for the elastomer composition. Use of coupling agents for such purpose is well known to those having skill in such art. The coupling agent typically has a moiety reactive with hydroxyl groups (e.g.: silanol groups) on the surface of the silica and another moiety interactive with the elastomer(s) to create the silica-to-rubber coupling effect.
For example, a coupling agent may be a bis-(3-trialkoxysilylalkyl) polysulfide where the polysulfide bridge contains an average of from about 2 to about 4, preferably from 2 to 2.6 or 3.5 to 4, connecting sulfur atoms. The alkyl groups may be selected, for example, from methyl, ethyl and propyl radicals. Exemplary of such coupler might be, for example, a bis-(triethoxysilylpropyl) polysulfide.
In further accordance with this invention, a pneumatic rubber tire is provided having at least one component as the said rubber composition.
In particular accordance with this invention, a pneumatic tire is provided having a tread of said rubber composition.
In practice, the segmented styrene/isoprene/butadiene terpolymer elastomer might be prepared, for example, by terpolymerizing the styrene, isoprene and 1,3-butadiene monomers in an organic solvent such as, for example, hexane, in the presence of a catalyst such as, for example n-butyllithium modified with TMEDA. The terpolymer is caused to be of a segmented configuration of styrene/butadiene copolymer segments and cis 1,4-polyisoprene segments, instead of a random configuration of styrene/isoprene/butadiene by sequential addition of first isoprene to create a relatively high cis 1,4-polyisoprene, (about 50 to about 90 percent cis 1,4 content), and then later the styrene and butadiene to create the segments.
It is readily understood by those having skill in the art that the rubber compositions would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents and reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur-vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
Typical additions of reinforcing carbon black have been hereinbefore discussed. Typical amounts of tackifier resins, if used, may comprise about 0.5 to about 10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids may comprise 1 to 20 phr. Such processing aids can include, for example, aromatic, napthenic, and/or paraffinic processing oils. Silica, if used, has been hereinbefore discussed. Typical amounts of antioxidants comprise about 1 to about 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in The Vanderbilt Rubber Handbook (1978), pages 344-346. Typical amounts of antiozonants comprise about 1 to about 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 2 to about 6 phr. Typical amounts of waxes comprise about 1 to about 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers comprise about 0.1 to about 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide. The presence and relative amounts of the above additives are considered to be not an aspect of the present invention which is more primarily directed to the utilization of unique blends of specialized elastomers in various tire components.
The vulcanization is conducted in the presence of a sulfur-vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur-vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur-vulcanizing agents are used in an amount ranging from about 0.5 to about 4 phr, with a range of from about 0.5 to about 2.25 being preferred.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally, a primary accelerator is used in amounts ranging from about 0.5 to about 2.0 phr. In another embodiment, combinations of two or more accelerators are used in which the primary accelerator is generally used in the larger amount (0.5 to 2 phr), and a secondary accelerator which is generally used in smaller amounts (0.05-0.50 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators have been known to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce satisfactory cures at ordinary vulcanization temperatures. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
The tire can be built, shaped, molded and cured by various methods which will be readily apparent to those having skill in such art.
The prepared tire of this invention is conventionally shaped and cured by methods known to those having skill in such art.